This invention relates to compositions for reducing the oxygen concentration present in an atmosphere or liquid (often referred to as oxygen scavenging). In one particular application, the compositions are used in or in association with food packaging.
A wide variety of foods and other materials are susceptible to loss in quality during storage under atmospheric levels of oxygen. The damage can arise from chemical oxidation of the product, from microbial growth, and from attack by verminxe2x80x94much of which may be avoided by reducing the oxygen availability in the environment of the materials. In the field of packaging, relatively low-oxygen atmospheres have traditionally been generated by vacuum packing and inert gas flushing. Such methods are not, however, generally applicable for various reasons. For example:
soft porous foods such as cakes cannot be subjected to strong vacuum;
fast filling speeds generally preclude substantial evacuation of or thorough inert gas flushing of food packages;
filling some gas-flushed containers, such as beer bottles often results in occlusion of air;
evacuation or flushing offers no residual capacity for removal of oxygen, which may have desorbed from the food or entered the package by leakage or permeation.
As a consequence there has been much interest in chemical techniques for generating low-oxygen atmospheres and deoxygenating liquid or semi-liquid foods. Thus, there are approaches based on the use of oxidisable solids, for example porous sachets containing iron powder. In another technique, oxidisable MXD-6 Nylon is blended with polyester in the walls of blow-moulded containersxe2x80x94the effectiveness of this depends on the presence of a cobalt salt catalyst, moreover the speed of oxygen removal is limited by the oxygen permeability of the polyester. Further methods include sandwiching crystalline oxidisable material between the layers of multilayer containers, and including a catalyst for the reaction of oxygen with hydrogen in a sandwich arrangement as above or as a deposit on the inner surface of the package.
Heterogeneous systems such as described above do not, however, adequately meet the general needs of the packaging industry, largely because they are often oxygen-sensitive prior to use or can be activated only under restricted conditions of, for example, temperature or humidity. U.S. Pat. No. 5,211,875 proposes a composition intended to avoid the problem of oxygen-sensitivity prior to use, involving an oxidizable organic compound (typically 1,2-polybutadiene) and a transition metal catalyst (typically cobalt salt). Oxygen scavenging is initiated by exposing the composition to an electron beam, or ultraviolet or visible light.
However, the inclusion of a transition metal catalyst has a number of disadvantages including added cost, solubility difficulties, and a xe2x80x9cgrittyxe2x80x9d appearance and reduced transparency of films made from such compositions. Some transition metal catalysts are also considered toxic and may not, therefore, be used with food.
The present invention avoids the disadvantages of including a transition metal catalyst. It may be based on plastic or other polymer-based compositions which can be activated as required, to effect reduction of ambient oxygen levels.
Accordingly, in a first aspect the present invention provides a composition for reducing the concentration of molecular oxygen present in an atmosphere or liquid, comprising at least one reducible organic compound which is reduced under predetermined conditions, the reduced form of the compound being oxidizable by molecular oxygen, wherein the reduction and/or subsequent oxidation of the organic compound occurs independent of the presence of a transition metal catalyst.
Preferably, the reduction and/or subsequent oxidation of the at least one reducible organic component is also independent of the presence of an alkali or acid catalyst.
The reducible organic compound for use in this invention may be reduced under predetermined conditions such as by exposure to light of a certain intensity or wavelength or, alternatively, by the application of heat, xcex3-irradiation, corona discharge or an electron beam. Possibly, also, the compound may be reduced by incorporating in the composition a reducing agent which in turn can be activated under predetermined conditions, say by heating.
Typically the reducible organic compound will be a compound having the capacity to be converted to an excited state such as a triplet form, which then becomes reduced to an essentially stable state by gaining or abstracting an electron or hydrogen atom from other molecules or by redistributing an electron or hydrogen atom within the compound itself. The reduced molecule is reactive towards molecular oxygen to produce activated species such as hydrogen peroxide, hydroperoxy radical or a superoxide ion. Preferably, the reducible organic compound is stable in air at room temperature or is in its fully oxidized state. Examples of suitable compounds include quinones, such as benzoquinone, anthraquinone (preferably, 9,10-anthraquinone) and naphthoquinone (preferably, 1,4-napthoquinone); and photoreducible dyes and carbonyl compounds which have absorbance in the UV spectrum, such as azo, thiazine, indigoid and triarylmethane compounds.
Most preferably, the reducible organic compound is a substituted anthraquinone such as 2-methylanthraquinone and 2-ethylanthraquinone. In some applications, 2-ethylanthraquinone shall be preferred to 2-methylanthraquinone due to its greater solubility.
The reducible organic compound component may comprise 0.1-99.9 wt % of the composition. More preferably, the reducible organic compound comprises 0.1-50 wt % of the composition.
Compositions of this invention which involve the formation of an activated oxygen species (eg, peroxide) may further comprise a scavenging component reactive towards the activated species. This may be embodied in the reducible organic compound itself, for example a quinone having an amine group would be effective, but in any event it should be an agent which is substantially stable in contact with air at room temperature. Suitable examples of the activated oxygen scavenging component include antioxidants such as alkylated phenols and bisphenols, alkylidene bis-, tris- and polyphenols, thio- and bis-, tris- and polyalkylated phenols, phenol condensation products, amines, sulfur-containing esters, organic phosphines, organic phosphites, organic phosphates, hydroquinone and substituted hydroquinones; inorganic compounds such as sulphates, sulfites, phosphites and nitrites of metals, particularly those of groups 1 and 2 of the periodic table and first row transition metals, zinc and tin; sulfur-containing compounds such as thiodipropionic acid and its esters and salts, thio-bis(ethylene glycol xcex2-aminocrotonate), as well as the amino acids cysteine, cystine and methionine; and nitrogen-containing compounds capable of reacting with activated forms of oxygen include primary, secondary and tertiary amines and their derivatives including polymers.
Preferably, the scavenging component reactive towards the activated oxygen species is selected from the group consisting of triphenylphosphine, triethylphosphite, triisoproppylphosphite, triphenylphosphite, tris(nonylphenyl) phosphite, tris(mixed mono- and bis-nonylphenyl) phosphite, butylated hydroxytoluene, butylated hydroxyanisole, tris(2,4-di-tert-butylphenyl) phosphite, dilaurylthiodiprpionate, 2,2-methylene-bis-(6-t-butyl-p-cresol), tetrakis(2,4-d-tert-butylphenyl)4,4xe2x80x2-biphenylene diphosphonite, poly(4-vinylpyridine) and mixtures thereof.
The activated oxygen species-scavenging component may be in the form of a polymer or oligomer. Such forms may be prepared by, for example, covalently bonding a compound such as those activated oxygen species-scavenging compounds listed above to a monomer or co-monomer. A limitation on the molecular size of the activated oxygen species-scavenging component will be the effect, if any, it has on functional properties of any other polymer with which it is combined as in blending for instance.
The activated oxygen species-scavenging component may comprise 0.1 to 99.9 wt %, more preferably, 0.1 to 50 wt % of the composition.
As an alternative to components which can be excited to a state which converts oxygen to an activated species, compositions according to this invention may comprise components which are excitable to a state in which they react and bind directly with oxygen diffusing into the composition from the surroundings.
The compositions according to the invention may further comprise an adhesive (eg, a polyurethane such as LAMAL) and/or a polymer. Preferred polymers are homogenous and include polyvinyls, polyolefins and polyesters or their copolymers, ethyl cellulose and cellulose acetate. Heterogeneous substrates, eg inorganic polymers such as silica gel or polymer mixtures may also be used.
Alternatively or additionally, the reducible organic compound itself maybe in a polymerised form either as homopolymers or copolymers. Oligomer forms may also be suitable. Reducible monomers can be made by covalently bonding an ethylenically unsaturated group to a reducible organic compound. The reducible organic compound may also carry groups capable of reaction with other polymerisable molecules and preformed polymers. Particular examples of ethylenically unsaturated reducible monomers include vinyl and isopropenyl derivatives, preferably bonded to the reducible organic compound in such a manner as not to decrease the lifetime of the triplet excited state compared with that of the unsubstituted reducible organic compound. Thus in the case of 9,10-anthraquinone substitution occurs preferably at the 2, 3, 6 or 7 positions. If such a reducible organic compound carries additionally further substituents besides the vinyl or isopropenyl group, such substituent should preferably be in one or more of the remaining preferred positions.
Co-monomers can be any ethylenically unsaturated substance whether mono-unsaturated, di-unsaturated or polyunsaturated. Examples include alkenes of carbon number two to eight, vinyl acetate, vinyl alcohol, acrylic monomers including methacrylic and acrylic acids, their amides, esters and metal salts as in ionomers, acrylonitrile, methacrylonitrile, norbornene, norbornadiene. If the reducible organic compound is a substituted 9,10-anthraquinone and is required to be difunctional monomer for formation of a polyester, the two carboxyl or hydroxyl substituents, or their derivatives should preferably be in any two of the positions, 2, 3, 6 or 7.
Reducible monomers may be polymerised as condensation polymers such as polyesters, including polycarbonates, polyamides, polyimides. An example of a polyamide is the polymer of 2,6-anthra-9,10-quinone dicarboxylic acid with 1,6-diaminohexane. Reducible monomers may also be polymerised with diisocyanates or diols to form polyurethanes or may be bonded to polyurethanes. An example of the latter is the reaction product of 2-bromomethyl-9,10-anthraquinone with the polyurethane from toluenediisocyanate and 1,6-hexandiol.
Preferably, the composition according to the invention comprises a reducible organic compound and an activated oxygen species-scavenging form, both of which are present in polymerised form(s).
Where the reducible organic compound is dispersed or dissolved in a polymer which does not readily donate a hydrogen atom or electron to the reducible organic compound in its excited state, an additional source of labile hydrogen or electron is preferred. Such a compound is preferably one containing a hydrogen bonded to nitrogen, sulfur, phosphorus or oxygen especially where a hydrogen is bonded to a carbon atom bonded to the abovementioned heteroatom. Alternative sources of electrons are salts of organic compounds such as the salts of sulfonic acids or carboxylic acids. In one form of the invention the sodium sulfonate salt of a polymerised 9,10-anthraquinone would be used. Thus the reducible organic compund itself can be the source of its own electron for the reduction process.
The reduced form of the organic compound used in the composition, brings about a reduction in the molecular oxygen concentration in the atmosphere or liquid through its oxidation by the molecular oxygen, the reduction and/or oxidation being independent of the presence of a transition metal catalyst and, preferably, also independent of the presence of an alkali or acid catalyst. Nevertheless, transition metal compounds, alkaline and/or acidic agents may also be included in the compositions where they may effect the rate of oxygen scavenging or may enhance the reduction and/or subsequent oxidation of the organic compound. For example, ascorbic acid may be included in the compositions comprising anthraquinones as a photoreduction enhancer.
Reduction of the reducible organic compound may take place only when convenient. This might be, for example, when the composition is being made into or brought into association with packaging material or, alternatively and perhaps more usually, after formation of a package and prior to filling and sealing. Reduction may even be deferred until after sealing of the package.
Thus, in a second aspect, the invention provides a method for reducing the concentration of molecular oxygen present in an atmosphere or liquid, comprising exposing the atmosphere or liquid to a composition according to the first aspect and thereafter, reducing the reducible organic compound.
Alternatively, the invention provides a method for reducing the concentration of molecular oxygen present in an atmosphere or liquid, comprising exposing the atmosphere or liquid to a pre-reduced form of a composition according to the first aspect.
The compositions according to the invention may be used independently or as components of blends. They may take the form of a cross-linked polymeric matrix, as in a can lacquer, or be bonded to or absorbed onto an inorganic polymer, such as silica. They may be effectively applied as, or incorporated in, for example, bottle closure liners, PET bottles, liners for wine casks, inks, coatings, adhesives, films or sheets either alone or as laminations or co-extrusions, or they may take the form of pads, spots, patches, sachets, cards, powders or granules which may be attached to packaging material or located independently within a package.
Films comprising the composition according to the invention may be monolayer or multilayer laminate, and may be used on their own or may be affixed or applied to a solid substrate (eg, a solid packaging material). Where the film is a multilayer film, it is preferable that an outer layer is an oxygen barrier film, so that the film may be used in a manner such that only the layer(s) containing the reducible organic compound is exposed to molecular oxygen from the atmosphere or liquid for which a reduction in molecular oxygen concentration is required.
Films comprising the composition may also be used is as a chemical barrier to oxygen transmission through a packaging material. Thus if a packaging material has a certain oxygen permeability, the oxygen passing through it from the outside environment into a reduced oxygen content atmosphere within the package can be scavenged by the reducible organic compound. The composition can be dissolved or dispersed within the packaging material or can be placed adjacent to it as an additional layer on the inner side of the package.
In multilayer laminate films, an activated oxygen species-scavenging component may be provided in a seperate layer from the layer comprising the activatable component.
Film layers containing a reducible organic compound may be formed either from molten plastic compositions extruded to give a particular shape or dimensions or from a liquid state which gives the final solid layer by reaction, or evaporation of a volatile liquid. Plastic compositions will often be extruded at temperatures between 50xc2x0 C. and 350xc2x0 C. depending upon chemical composition and molecular weight distribution. Extrusion may be via a die to give a film layer either alone or as a component layer of a multilayer coextrusion. The layer comprising the reducible organic compound may be extruded onto another substrate as in extrusion coating and lamination. Extrusion may be followed by moulding as in injection or blow moulding. These processes can involve the formation of foams in some instances.
The composition according to the invention may also take the form of a printing ink, coating or lacquer. These may or may not be pigmented. The printing inks, coatings and lacquers will normally be applied in a liquid state and solidified by evaporation of the solvent or dispersion medium or by reaction of some of the constituents.
While the composition and methods according to the invention are likely to be of particular value in food-packaging situations where oxygen removal is desirable, their utility is not limited thereto. Other applications include, for example, the generation of low-oxygen atmospheres in vessels for anaerobic or microaerophilic microbiology, or the generation of low-oxygen gas for blanketing flammable or oxygen-sensitive materials. The technology can also be used in conjunction with technologies based on other means of oxygen scavenging such as photosensitized generation of carbon dioxide.
Compositions according to the invention may be re-reduced, if necessary, by resubjecting to the predetermined condition to recommence oxygen scavenging. This may be particularly useful if the composition has been exposed to air prior to package sealing. Re-reduction may be achieved at a light intensity as low as ambient room illumination for an hour depending upon the amount of re-reduction required.
In addition to the advantages disclosed above, the method can, in some instances, be practised with compositions formulated to be self-indicating in respect to their capacity for oxygen removal. That is, some reducible organic compounds upon reduction will undergo a change in colour or change in UV-visible, infrared or near-infrared absorption spectrum. For example, photoreduction of quinones and some of their derivatives results in a spectral shift from the UV to longer wavelengths, especially to the visible region of the spectrum; by incorporation of such compounds, package material can be formulated which will undergo a colour change as the capacity for reducing the oxygen concentration becomes exhausted.
This colour change also provides a mechanism for checking whether all of the reducible organic compound in the composition has been reduced. Where reduction is found to be incomplete, the composition may be resubjected to the predetermined conditions. Further, such compositions may also be used as an indicator of seal breakage. That is, in the area of film where a heat seal or other seal is made between the film containing the reducible organic compound and a material of sufficiently high oxygen barrier, oxygen cannot reach the reducible organic compound as fast as it can in other areas. The seal area therefore remains coloured due to the presence of reduced organic compound. The fluorescent emission from the reduced organic compound is particularly useful for this purpose. A green fluorescence is seen when 9,10-anthraquinone with substitutents in the 2-position are bonded by a methylene group to the ring. Alternatively, a strip or ring of the composition may be located on the inner side of the sealed package adjacent to the seal. Where the seal is formed by an adhesive, the composition may comprise the adhesive. If the seal should be incomplete or become broken in any way, then this may be detected by a colour change in the composition.
Visible colour changes may be detected by eye, whereas changes in UV-visible, infrared or near infrared absorption spectrum may be measured with an appropriate device such as a photocell used with a light source of appropriate wavelength and intensity.
The invention will now be further described with reference to the following non-limiting examples and figures.